In a computer network, network switching devices (switches) interconnect to form a path for transmitting information between an originator and a recipient. A routing mechanism, or protocol, defines switching logic that forwards the transmitted information in the form of packets between the switches as a series of “hops” along a path. At each switch, the switching logic identifies the next switch, or hop, in the path using an identifier such as a Media Access Control (MAC) address. Shortest Path Bridging (SPB) is a routing mechanism having switching logic such that each switch advertises the nodes it knows about to all the other switches, and eventually all the switches in the network have the same picture of the network and therefore can forward frames to the next hop along a shortest path.
SPB is defined in IEEE-802.1aq: IEEE standard for Shortest Path Bridging, and operates in conjunction with IEEE-802.1ah: IEEE standard for Provider Backbone Bridging, sometimes referred to as Mac-in-Mac encapsulation. Both SPB and SPBM forward packets on shortest path trees with minimum path cost as a first order tie-breaker, where for any pair of nodes A and B, the unicast path for A to B is the exact reverse of the path from B to A (reverse path congruency), and all multicast traffic between the two nodes follows the unicast path (multicast and unicast congruency). These are extensions to fundamental Ethernet forwarding properties in IEEE bridged networks.
SPB technology allows a network administrator to easily form mesh networks that distribute load more evenly across the network topology since it can mitigate bottlenecks at core links for traffic that only needs to go from one distribution switch to another. SPB technology is being adopted in Ethernet based data networks to enable Layer-2 and Layer-3 network virtualization. These networks are expected to continue to deliver business critical services even when a variety of network faults occur (or when maintenance operations are performed on the network).
Multi-homing is a mechanism by which an access network connects to and uses two or more devices in the transport network as if it were connecting to a single device. The multiple devices (network switches) in the transport network exchange information between them which allow them to present the access network to the rest of the transport network as if the access network was connected to a single device in the transport network. Failure of the connection of one of the transport devices to the access network or even the complete failure of one of the transport devices will not cause loss of connectivity between the access network and the transport network. The access network therefore exhibits multi-homed access, which is an access network that uses multi-homing to connect to multiple transport devices, and the transport devices define a multi-homed edge, or a group of partner devices, in the transport network that provide multi-homing service to an access network.
One conventional model for providing dual homing uses an access Link Aggregation Group (LAG) connecting two SPB Edges Nodes configured as a pair of InterSwitch Trunk (IST) switches. From a SPB Network perspective the pair of IST switches appear as two separate SPB Nodes. But from a forwarding plane perspective they appear as a single switch.
Another conventional model utilizes stacking with SPB. A set of SPB Edge Nodes provide a Distributed MultiLink Trunk (D-MLT) connecting an access device to a SPB Network. There can be more than two nodes in the same stack. From an SPB network perspective the stack appears as a single switch from both a control-plane and forwarding plane perspective.